The main thrust of this research program is directed toward the elucidation of the factors involved in migration and differentiation of primordial germ cells, the progenitors of the future sex cells of the adult organism. In all vertebrates studied, including humans data demonstrate that the future germs cell arise outside of the gonad, migrate to and colonize the sex organs, and there differentiate into ova and spermatozoa. One of the objectives of the proposed research is to further explore the factors involved in the directed migration of germ cells to the developing gonads. Parabiosis between untreated and ultraviolet irradiated embryos, and implantation of donor germ cells in host embryos will be the approaches that we will continue to use in these experiments. Another objective will be to explore some of the factors involved in the differentiation of germs cells in the gonads. Nuclear transplantation between subspecies and species, and between embryos of different ploidies, in combination with the above cited methods will be utilized to analyze this aspect of the proposed research. The research project is of value for two reasons. First of all, there have been only a few explorations addressed to the question of how germ cells manage to find their way to the gonads. Secondly, an understanding of the mechanisms of germ cell migration and germ cell differentiation in vertebrates could contribute to the advancement of other practical approaches toward sexual sterility or fertility of individuals.